LibJS: Implement Iterator.prototype.find

Userland/Libraries/LibJS/Runtime/IteratorPrototype.cpp

@@ -41,6 +41,7 @@ ThrowCompletionOr<void> IteratorPrototype::initialize(Realm& realm)
     define_native_function(realm, vm.names.forEach, for_each, 1, attr);
     define_native_function(realm, vm.names.some, some, 1, attr);
     define_native_function(realm, vm.names.every, every, 1, attr);
+    define_native_function(realm, vm.names.find, find, 1, attr);
 
     return {};
 }
@@ -690,4 +691,51 @@ JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::every)
     }
 }
 
+// 3.1.3.12 Iterator.prototype.find ( predicate ), https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.find
+JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::find)
+{
+    auto predicate = vm.argument(0);
+
+    // 1. Let O be the this value.
+    // 2. If O is not an Object, throw a TypeError exception.
+    auto object = TRY(this_object(vm));
+
+    // 3. If IsCallable(predicate) is false, throw a TypeError exception.
+    if (!predicate.is_function())
+        return vm.throw_completion<TypeError>(ErrorType::NotAFunction, "predicate"sv);
+
+    // 4. Let iterated be ? GetIteratorDirect(O).
+    auto iterated = TRY(get_iterator_direct(vm, object));
+
+    // 5. Let counter be 0.
+    size_t counter = 0;
+
+    // 6. Repeat,
+    while (true) {
+        // a. Let next be ? IteratorStep(iterated).
+        auto next = TRY(iterator_step(vm, iterated));
+
+        // b. If next is false, return undefined.
+        if (!next)
+            return js_undefined();
+
+        // c. Let value be ? IteratorValue(next).
+        auto value = TRY(iterator_value(vm, *next));
+
+        // d. Let result be Completion(Call(predicate, undefined, « value, 𝔽(counter) »)).
+        auto result = call(vm, predicate.as_function(), js_undefined(), value, Value { counter });
+
+        // e. IfAbruptCloseIterator(result, iterated).
+        if (result.is_error())
+            return *TRY(iterator_close(vm, iterated, result.release_error()));
+
+        // f. If ToBoolean(result) is true, return ? IteratorClose(iterated, NormalCompletion(value)).
+        if (result.value().to_boolean())
+            return *TRY(iterator_close(vm, iterated, normal_completion(value)));
+
+        // g. Set counter to counter + 1.
+        ++counter;
+    }
+}
+
 }

Userland/Libraries/LibJS/Runtime/IteratorPrototype.h

@@ -32,6 +32,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(for_each);
     JS_DECLARE_NATIVE_FUNCTION(some);
     JS_DECLARE_NATIVE_FUNCTION(every);
+    JS_DECLARE_NATIVE_FUNCTION(find);
 };
 
 }

Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.prototype.find.js

@@ -0,0 +1,113 @@
+describe("errors", () => {
+    test("called with non-callable object", () => {
+        expect(() => {
+            Iterator.prototype.find(Symbol.hasInstance);
+        }).toThrowWithMessage(TypeError, "predicate is not a function");
+    });
+
+    test("iterator's next method throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                throw new TestError();
+            }
+        }
+
+        expect(() => {
+            new TestIterator().find(() => 0);
+        }).toThrow(TestError);
+    });
+
+    test("value returned by iterator's next method throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                return {
+                    done: false,
+                    get value() {
+                        throw new TestError();
+                    },
+                };
+            }
+        }
+
+        expect(() => {
+            new TestIterator().find(() => 0);
+        }).toThrow(TestError);
+    });
+
+    test("predicate function throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                return {
+                    done: false,
+                    value: 1,
+                };
+            }
+        }
+
+        expect(() => {
+            new TestIterator().find(() => {
+                throw new TestError();
+            });
+        }).toThrow(TestError);
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Iterator.prototype.find).toHaveLength(1);
+    });
+
+    test("predicate function can see every value", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+        }
+
+        let count = 0;
+
+        const result = generator().find((value, index) => {
+            ++count;
+
+            switch (index) {
+                case 0:
+                    expect(value).toBe("a");
+                    break;
+                case 1:
+                    expect(value).toBe("b");
+                    break;
+                default:
+                    expect().fail(`Unexpected reducer invocation: value=${value} index=${index}`);
+                    break;
+            }
+
+            return false;
+        });
+
+        expect(count).toBe(2);
+        expect(result).toBeUndefined();
+    });
+
+    test("iteration stops when predicate returns true", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+            yield "c";
+        }
+
+        let count = 0;
+
+        const result = generator().find(value => {
+            ++count;
+            return value === "b";
+        });
+
+        expect(count).toBe(2);
+        expect(result).toBe("b");
+    });
+});